The disclosed invention is directed generally to holographic exposure systems, and more particularly to a holographic exposure system that includes a holographic prism for efficiently coupling a hologram constructing beam to a hologram recording layer.
Holographic exposures generally involve two coherent construction beams that are directed to a holographic recording medium in which the beams constructively interfere. Pursuant to factors such as reconstruction angle, reconstruction wavelength, viewing angle, and construction wavelength, a relatively large angle of incidence relative to normal for one or both of the construction beams is frequently required. While the simplest way of achieving a relatively large angle of incidence is to direct a construction beam to the hologram recording layer at the relatively large angle, coupling of the beam into the hologram recording layer decreases as angle of incidence increases. A common technique for coupling a construction beam of relatively large incidence angle, particularly if the desired angle in the hologram recording layer is greater than the critical angle, is to use an appropriately shaped prism having an entrance facet that provides a relatively small angle of incidence for the construction beam and another facet that is index matched to the holo-gram recording layer and through which the construction beam exits at a relatively large angle relative to normal to the hologram recording layer. The other construction beam will typically also be coupled to the hologram recording layer by the prism, for example, by a separate entrance facet or the same entrance facet utilized for the other construction beam, in which case the angle of the entrance facet may be compromised so as to be non-optimum for both beams. The prism can be a glass or plastic prism, or it can be a glass tank filled with fluid.
Considerations with the use of refractive prisms to couple construction illumination to a hologram recording medium include the fact that a glass prism can be large and therefore expensive, limitations on exposure system design that may be imposed by the physical size of a refractive prism, as well as the inconvenience of utilizing a large prism. Moreover, large fluid filled prisms tend to have striation problems, and large glass prisms can have birefringence problems.